Find the line where segmentation fault occurs

With using gcc, new users generally gets a segmentation fault while doing operations with pointers in C. As gcc is the most robust compiler of C, it doesn't allow to do anything wrong with memory references. Your TurboC++ compiler may allow invalid memory references in many cases but gcc may not. But, don't be scared of it. By using one of the best debuggers 'gdb' will find the line where core dump- segmentation fault occurs.

There are six reasons for occurrences of segmentation fault in C program. Out of all, generation of SIGSEGV is most frequent event which leads segmentation fault. Consider following example:

    #include<stdio.h>

    int main()

    {

       int x,*y,*z;

       x=10;

       *y=12;

       *z=x+*y;

       printf(”Addition is %d”,*z);

       return(0);

    }

This program will work fine in TurboC++ compiler but gcc will produce segmentation fault. As, it is a run time error the line where return fault has occurred, won't be displayed. So, what to do..?

Let see find it. First compile the program as,

gcc myprogram.c -g

The program will be compiled successfully and it will generate a.out file which is executable. We may use -o option also to give another name to output file. The -g option here gives ability to output file to get debugged. Now, the debugger will be enabled for output file a.out . Use gdb (GNU debugger) to debug program.

gdb a.out

This will start debugging and gdb> prompt will be shown. Use 'break' instruction to insert breakpoint in the program. Such as,

gdb> break 1

It will insert breakpoint at line no.1 in the program. Now, run the program by 'run' command.

gdb> run

This will execute your program till first breakpoint then after, we may step through our program by 'next' command. The 'next' command will execute the program line by line.

Wherever you find the segmentation fault, the program will stop on that line it will also show the line number. The above program will stop the execution due to segmentation fault on line no.6. It is invalid memory reference which is not allowed by gcc!

Swapping Techniques

Swapping refer to exchanging the contents of the variables. It is most important process in the programming. Generally, the sorting techniques use the swapping operations. When the information is very vast, we need some efficient sorting techniques to improve the speed of processing. You should use the swapping technique which has least time and space complexity in order to improve the speed of processing of the algorithms.
The generalized swapping technique that most software developers use it is have a temporary variable. For example:

if,
x = 12
y = 15

t = x
x = y
y = t

This will swap the contents of variables x and y. But, this technique wastes the extra memory in variable 't'. We can eliminate this by using the concept of addition and subtraction to swap the contents. For example:

x = x + y
y = x - y
x = x - y

Here, we are not using the temporary variable but three mathematical operations are required. This can be optimized by introducing the bitwise operation technique to swap the contents. The XOR operation will do the task for us. Let's see how this technique work.

You might be knowing the truth table of XOR operation.

x   y   x XOR y
0   0      0
0   1      1
1   0      1
1   1      0

The following operation will do the swapping.

x = x XOR y
y = x XOR y
x = x XOR y

Lets demonstrate it by simple example.

Let x = 12 and y = 15
means in digital or binary format, x = 1100 and y = 1111

so,

x = x XOR y = 1100 XOR 1111 = '0011'
y = x XOR y = 0011 XOR 1111 = '1100'
x = X XOR y = 0011 XOR 1100 = '1111'

So the final value of x and y are 1111 (15) and 1100 (12) respectively!

You may check this technique in C/C++/Java program by following operations.

int x = 12, y = 15;
x = x ^ y;  /* The ^ called as XOR operator */
y = x ^ y;
x = x ^ y;

These operations can be combined using assignment operator also. Such as,

x^=y^=x^=y;

Done.

tushar@tusharkute.com

DDD: Best Visual Debugger

Debugging is the most important activity in the software and program development. In order to remove or identify the logical errors in the program; debugging is done. Linux uses the utility of ‘gdb’ (GNU Debugger) to debug the programs. It has a strong instruction set but lacking in visual GUI debugging utility. 

 

DDD or Data Display Debugger is a visual debugger made available under GNU public license uses the gdb activities with GUI at foreground! Not only gdb but ddd has in-built command-line debuggers such as DBX, JDB, HP Wildebeest Debugger (WDB), XDB, the Perl debugger, the Bash debugger, the Python debugger, and the GNU Make debugger etc. So, by this visual debugger you can debug programs of C, C++, Java, Shell, Perl, Python etc.

 

Here I am giving a small tutorial to show how to use ddd to debug the C programs. The advanced tutorials will be given afterwards.

 

Let’s start… As I told ddd is available under GPL, so you can download the debugger using following command directly by debian version Linux.

 

sudo apt-get install ddd

 

or you may go to ‘Ubuntu Software Centre’ and search for ‘ddd’ as shown in the picture 1 below. You may get information about it and install from there.

[Note: If images are not visible, click on it to view large] 

 

Picture 1: DDD entry in Ubuntu Software Centre

For example, you have written a C program code named ‘basic.c’. Compile it using gcc in the following fashion:

 

gcc –g –o basic basic.c

 

here, -o suggests the name of the output file that you will create after compilation of ‘basic.c’ and –g suggests that you are giving ability to debug this program. If the –g is missing your program can’t be debugged by gdb or ddd. After compilation you will get an executable file named ‘basic’ (without extension). Note: you may give any name to your executable file at the time of compilation after option –o.

 

Your program will be compiled successfully as shown in the picture 2 below.

 

Picture 2: Compilation of program for debugging

Now, open your ‘ddd’ by simply typing ‘ddd’ on the terminal.

 

Open your program ‘basic.’c by using ‘File’ menu -> open program option. Your program will be opened as shown in the picture 3 below.

 

Picture 3: The ddd windows

Your program will be visible in ‘Program Window’. Below of it, you may find ‘Assembly Code Window’, where the assembly equivalent code is shown. And below of this window, your gdb command line debugger is shown. You may instruct program directly by using gdb also! 

 

In order to set a breakpoint inside the program move your mouse on to respective line and press right click. You may get options as shown in the picture 4 below.

Picture 4 : Setting a breakpoint

The any number of breakpoints can be set in a single program. It is used for executing the program till that point. From that line onwards, you may execute program step-by-step. The breakpoints in the function can be set by typing the command ‘b main’ in the ‘gdb window’ also! Or ‘b 12’ can also be written. Here 12 is the line number in the program.

Now, go to ‘Program’ menu and click on the ‘Run’. Your program will get executed till the first breakpoint set. See the picture 5 below:

Picture 5: Watch variable values by F6

The green arrow shows the line which is in execution currently. Red dot shows the breakpoint. Now, from here onwards you may execute step-by-step just by pressing function key ‘F6’. The program execution will be processed step-by-step as you go on pressing ‘F6’. You may find arrow is traversing with the lines of execution. When you move the mouse on any variable used in the program, you may get the value of the variable on current execution. In the picture 5, you may see the values inside array ‘a’ is shown by the ‘ddd’ at mouse position. By this way you can check the contents of the variable at any point in the program.

 

You may check the content by typing the command in the ‘gdb window’ as shown in the picture 6 below. It has shown the contents of a[i] by typing ‘print a[i]’ in the gdb window.

Picture 6: Use gdb window.

The features of ‘ddd’ are not limited with this. It has a rich set of applications. Just find the options given in the ‘ddd’s menu. You will learn a lot!